17 Mar 2015

Electron Counting and CBC Assignments for Organometallic Complexes

In-Class Activity

Submitted by Matt Whited, Carleton College
Categories
Description: 

This in-class group activity provides several examples of varying difficulty for students to assign MLXZ classifications and electron counts to organometallic complexes.  Though some of the problems are straightforward, some are really ambiguous, and the intent is for student groups to grapple with the issues raised by each one and present their findings to the class to spark further discussion.

Learning Goals: 

* Students should be able to use the covalent bond classification method to assign MLXZ classifications to a variety of organometallic complexes.

* Students should be able to defend their assignments using both organic and inorganic views of structure and bonding.

* Students will understand the ambiguities associated with assigning bond orders, valencies, oxidation states, etc., with the hope that their understanding of covalently bonded organometallic systems will become more nuanced.

Implementation Notes: 

I split students into groups of 3-4, as noted in the handout.  Each group was assigned one or two problems that they would need to present to the rest of the class, but all groups were encouraged to work on all of the problems to facilitate discussion later on.

After about 15 minutes, I had the student groups present and encouraged questions and challenges to their assignments.  We had in-depth discussions of problems 8 and 9, which are clearly the most ambiguous.

Time Required: 
45 minutes (can be less, depending on how structured)
Evaluation
Evaluation Methods: 

Students were evaluated informally as I walked around to help the groups as well as during presentations.

Evaluation Results: 

A large majority of the students had no problem making assignments for the simple and intermediate cases.  This outcome is largely a testament to the ease of use of the CBC method.  In fact, students who had no background in inorganic or organometallic chemistry tended to perform a little better because they were less likely to bring in preconceptions about "oxidation state".

For the more difficult problems, I found that students did not have the nuance to be able to come up with good descriptions for the delocalization of charge from Fe onto the bridging N2 unit in #8.  However, they engaged in a lively discussion with me about how we should treat the molecule (this was just a week before Pat Holland came to Carleton to present his research so it was timely!).  On #9, we also had a good discussion because several of the more advanced (and even a few less advanced) students wanted the cycloheptatrienyl ligand to be aromatic but could not figure out how to make it work with the MLXZ system.  As I note, the answer is ambiguous, and I think I disagree with Parkin about the best way to assign it (see the linked J Chem Educ article for his description of a similar system on Ti).  Nevertheless, CBC carried the day and I found that the students with little to no background did extremely well on the next electron-counting homework assignment.

Creative Commons License: 
Creative Commons Licence

Comments

This looks like a great combination of fun and evil. I look forward to using it in the fall with my seniors.

Hello Matt Whited:

Congratulations on putting together an excellent activity for the CBC method. I really liked all your examples. The last one is a real twister. I think the reason is as follows.

The MO diagram of a symmetrical C7H7+ has a degenerate set of orbitals with only one electron. This is a clear case for Jahn-Teller distortion. After undergoing distortion, the molecule will now have an empty orbital LUMO and a lower lying radical SOMO as the highest occupied orbital.  So we will now have a L3X system. 

The artificial way of making cyclohexatrienyl unit into L2X3 has no basis in MO as the CBC web site suggests. I am compiling a set of examples where better assignments can be made. I will make it available for VIPER members eventually.

Samuelson (Indian Institute of Science, Bangalore, INDIA.)

 

 

I don't think the assignment of the cycloheptatrienyl is ambiguous. I will admit that this is not a system I spend a lot of time thinking about, but fortunately I know someone that has spend a good amount of time considering it. The problem with just considering cycloheptatrienyl as a stand alone radical is that it is completely ignoring how it interacts with the metal when it is bonding. For example, cyclobutadiene is much more square when it is bonded to a metal. The experimental evidence supports that cycloheptatrienyl is best considered L2X3.

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